Steam generators, both those that are nuclear powered and those that are fired by other power sources, are subject to the build-up of sludge which may form concentration sites for contaminating chemical impurities adjacent to the steam generating structures in the generator. These contaminants, which include, for example, chlorides, sulfides and caustics, may become sufficiently concentrated to damage the steam generator tubes. Consequently, the generator must be cleaned periodically to prevent the concentration of corrosion-causing chemical contaminants in the steam generator and the resulting corrosion of generator components.
One well-known steam generator cleaning process is a two-step chemical descaling process based on the dissolution and chelation of iron and copper, which are the major components in a copper-bearing generator sludge, with ethylenediaminetetraacetic acid (EDTA). Magnetite iron, which includes both Fe.sup.+3 and Fe.sup.+2, reacts with EDTA as follows: EQU Fe.sub.3 O.sub.4 .revreaction.Fe.sub.2 O.sub.3 +FeO EQU Fe.sup.+3 +EDTA.sup.-4 .revreaction.FeEDTA.sup.- EQU Fe.sup.+2 +EDTA.sup.-4 .revreaction.FeEDTA.sup.-2
Copper reacts with EDTA after being oxidized by hydrogen peroxide as follows: EQU Cu+H.sub.2 O.sub.2 .revreaction.Cu.sup.+2 +H.sub.2 O+1/2O.sub.2 EQU Cu.sup.+2 +EDTA.sup.-4 .revreaction.CuEDTA.sup.-2
In this process, the temperature of the copper solvent is significantly lower than that of the iron solvent to minimize decomposition of the oxidant and corrosion effects. An initial rinse is followed by an initial solvent exposure, which can be either the copper or iron solvent. The solvent exposure is repeated until analyses performed on samples from the process solution show iron, copper, EDTA and/or hydrogen peroxide levels to be concomitant with desired termination levels. A rinsing step follows, and then a different solvent exposure is performed, except that two rinses are required after the iron solvent to help achieve the 100.degree. F. (37.8.degree. C.) cooldown required before the copper solvent step can be performed. A passivation rinse completes the process to form protective oxide films on the surfaces of steel components.
One difficulty with this process is that the iron cleaning solvent tends to cause corrosion of carbon and low alloy carbon steel generator components. A limited amount of corrosion, however, has been determined to be an acceptable trade-off because of the effectiveness of the cleaning process. Another difficulty presented by the aforementioned chemical descaling process is the disposal of the chelating materials used and generated by the process. These chelating materials are not accepted at low level radioactive waste disposal sites, primarily because of their high EDTA content. In addition, the chelating agents are capable of radioactive metals out of the waste which could end up in ground water.
U.S. Pat. No. 4,632,705 to Baum discloses a process for cleaning deposits from the restricted areas of a steam generator of a nuclear power plant system which overcomes, to a large extent, the corrosion problem by increasing the concentration of an aqueous organic cleaning agent solution in the specific areas to be cleaned by varying the temperature and pressure of the cleaning solution. However, this patent does not suggest processing the cleaning solution to recover the cleaning agent to facilitate its disposal or reuse. Consequently, disposal of the contaminated cleaning solution continues to remain a problem.
U.S. Pat. Nos. 4,681,705 to Robertson and 4,693,833 to Toshikuni et al. both disclose methods of treating radioactive liquids in the course of operating and cleaning nuclear power facilities. U.S. Pat. No. 4,681,705 is specifically directed to the decontamination of mixtures of water and water-immiscible organic liquids, such as contaminated reactor lubricating oil. A water-soluble chelating agent, such as EDTA, and, optionally, a water soluble inorganic precipitating agent are used for this purpose. The acidity is adjusted to promote the chelating action desired, which is preferably the removal of Cobalt-60, characteristically the most difficult radionuclide to remove. The optimum pH for the removal of Cobalt-60 is greater than 7, with the best results achieved at a pH of about 10.5. The decontaminated oil is disclosed to be suitable for disposal by burning, while the chelated radionuclide-containing solution is stated to be disposed of by conventional methods. However, no provision is made for recovery or reuse of the chelating agent.
The Toshikuni et al. patent discloses a method of treating radioactive waste water containing organic materials generated during chemical decontamination of nuclear power facilities. This method decomposes the decontaminating agents, which are mainly organic acids, by high efficiency oxidation in the presence of metal ion catalysts. Rapid decomposition of these organic acids occurs at temperatures of 60.degree. to 90.degree. C. with H.sub.2 O.sub.2 in the presence of copper ions or copper and iron ions. However, this patent is completely silent regarding the disposal of the radioactive components of the waste water or the recovery or disposal of the decontaminating agents.
U.S. Pat. No. 3,506,576 discloses a cleaning solution useful for cleaning ferrous based metal surfaces, such as those of steam boilers, which is an aqueous alkaline solution of a strong chelating agent, for example EDTA, that contains a water soluble sulfide capable of providing sulfide ions. The cleaning solution additionally prevents the deposition of copper on the ferrous metal. However, there is no suggestion that the EDTA present in this cleaning solution could be recovered for reuse following the chemical decontamination of a nuclear-fired steam generator.
The prior art has failed, therefore, to provide a process which produces maximum recovery for reuse of the cleaning components used in the cleaning of steam generators. The prior art has further failed to provide a process for the recovery of nuclear steam generator cleaning agents which allows recovery of the cleaning agents in a form that permits their reuse and which also allows the separation of radioactive components from the cleaning agents in a form acceptable for waste disposal.